This invention relates to protective coatings and, more particularly, to the processing of an oxide/phosphate coating to minimize physical defects therein.
In many aircraft gas turbine applications, articles are simultaneously exposed to elevated temperatures and oxidative/corrosive environments. The corrosion arises both from corrosive species such as salts that are ingested into the gas turbine with its air supply, and also corrosive species that are produced in the combustor when the ingested air is mixed with fuel and ignited. The corrosion of the articles in many cases is accelerated by the loads applied to the articles. The general trend in aircraft gas turbine engine design is toward higher operating temperatures and applied loads for improved fuel efficiency and performance. This trend leads to a greater severity of the oxidation and corrosion problems with increasing temperatures.
Protective coatings are applied to the articles to inhibit the environmental damage. A wide variety of protective coatings are used, according to whether the application involves exposure to air or to combustion gas, the temperature, the thermal excursions during service, whether wear occurs at the surface, and other factors. These protective coatings are usually applied to new articles, and then reapplied during repair and refurbishment.
One type of protective coating utilizes a multilayer arrangement of one or more oxide-based layers applied to the surface of the article substrate, and a phosphate layer applied over the oxide layer(s) as a sealant. After application, the protective coating is cured. This protective coating is relatively thin, on the order of 0.0005-0.0025 inches in thickness depending upon the selection of the types of layers.
The oxide/phosphate protective coating has good corrosion protection for its thickness, at temperatures of up to about 1400xc2x0 F. However, it has some shortcomings, and there is a need for an improved approach to such oxide/phosphate coatings. The present invention fulfills this need, and further provides related advantages.
The present invention provides a substrate protected by a protective coating of the thin, multilayer oxide/phosphate type, and a method for preparing the protected substrate. In the work leading to the present invention, the inventors have observed that the conventionally applied oxide/phosphate coatings often exhibit physical defects in the form of regions of small bubbles and local areas of coating spallation. These physical defects compromise the performance of the protective coating by serving as openings through the protective coating, through which the corrodants may penetrate to the article substrate. The present approach reduces the occurrence of these physical defects, producing a smooth, continuous protective coating of near-uniform thickness. The protective coating therefore provides excellent protection to the article substrate at intermediate service temperatures.
A method for protecting a substrate includes the steps of providing a substrate, such as a component of a gas turbine engine, and applying a multilayer protective coating to the substrate to form an initially coated substrate. The multilayer protective coating comprises an oxide layer (preferably comprising oxide particles in a binder such as a phosphate binder) and a phosphate layer overlying the oxide layer. The oxide layer may include a single layer, such as a chromium oxide layer, or it may include two or more sublayers, such as an aluminum oxide sublayer and a chromium oxide sublayer. The phosphate layer is preferably initially an inorganic phosphate in an organic binder. The multilayer protective coating is cured by degassing the multilayer protective coating of the initially coated substrate in a pre-cure degassing temperature range of from about 250xc2x0 F. to about 500xc2x0 F. for a time of at least about 30 minutes, and thereafter heating the multilayer protective coating to a curing temperature range of from about 1200xc2x0 F. to about 1400xc2x0 F. for a time of at least about 30 minutes. Optionally, there may be an additional step, after the step of degassing and before the step of heating, of maintaining the multilayer protective coating in a mid-temperature range of from about 500xc2x0 F. to about 1200xc2x0 F. for a time of more than about 30 minutes, and preferably from about 30 to about 90 minutes.
The degassing step may be performed by holding the multilayer protective coating at a pre-cure degassing temperature within the pre-cure degassing temperature range for a time of from about 30 to about 90 minutes. It may instead be performed by maintaining the multilayer protective coating within the pre-cure degassing temperature range (but not necessarily at the constant pre-cure degassing temperature) for a time of from about 30 to about 90 minutes. For example, the multilayer protective coating may be continuously heated through the pre-cure degassing temperature range but not held at any fixed temperature within that range.
Similarly, the optional step of maintaining the multilayer protective coating in the mid-temperature range may be performed by holding at a mid-range temperature in the mid-temperature range. It may instead be performed by maintaining the multilayer protective coating within the mid-temperature range (but not necessarily at any constant temperature value) for a time of from about 30 to about 90 minutes. For example, the multilayer protective coating may be continuously heated through the mid-temperature range.
This stepped heating approach allows gases and organic components of the initially applied protective coating to be gradually evolved through the protective coating and to the atmosphere, before the protective coating cures and hardens. The gases and organic components are therefore not trapped within the protective coating, leading to the physical defects discussed earlier. In the prior approach wherein curing was accomplished in a single step, typically by placing the initially coated substrate article in a furnace at an elevated temperature, the curing of the protective coating tended to trap the gases and organic components of the phosphate binder within the coating because they were evolved simultaneously with the curing and hardening of the protective coating. Since the protective coating is most rapidly heated at its surface, it tends to harden from the outside toward the inside of the protective coating, increasing the incidence of trapping of gases and organic components within the protective coating. The present approach allows the gases and organic components to evolve before the protective coating cures.
The resulting article substrate with its protective coating is therefore substantially free of physical defects in the protective coating. As a result, the points of weakness in the corrosion protection, associated with such physical defects, are minimized. The protection afforded by the protective coating is therefore more complete than that provided by the protective coating of the prior approach. The present heating procedure is slower than that conventionally used, but it leads to a better protective coating.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.
FIG. 1 is a schematic sectional view of a conventionally prepared oxide/phosphate protective coating;
FIG. 2 is a block flow diagram of a preferred approach for practicing the present invention;
FIG. 3 is a schematic sectional view of a first embodiment of an oxide/phosphate protective coating prepared by the present approach; and
FIG. 4 is a schematic sectional view of a second embodiment of an oxide/phosphate protective coating prepared by the present approach; and
FIG. 5 is a schematic graph of temperature as a function of time in the curing step.